Journal of Superconductivity and Novel Magnetism最新文献

Cobalt oxide (Co₃O₄) is a rare-earth-free spinel material with ferromagnetic ordering, making it a promising candidate for microwave and spintronics applications. This study investigates the controlled introduction of lattice defects through antimony (Sb) and samarium (Sm) doping in Co₃O₄ to tune its magnetic properties. Substitution of Sm and Sb is expected to generate oxygen vacancies (V_o) and modify cation distribution, thereby altering magnetic ordering. Structural refinement confirms that undoped Co₃O₄ crystallizes in a cubic spinel structure with space group F(stackrel{prime }{4})3m. Doping with Sb and Sm ions leads to traces of SbO₂. X-ray photoelectron spectroscopy (XPS) reveals that dopant incorporation disrupts the super-exchange pathways that govern spin orientation. This disruption weakens ferromagnetic interactions, as reflected in the softening of magnetic hysteresis and reduction of coercive fields in the doped samples. While pristine Co₃O₄ exhibits signatures of canted magnetic ordering at room temperature, doping modifies super-exchange strengths and bond angles, leading to further magnetic softening. Additionally, the dopants induce a structural transition from a normal spinel to a mixed-spinel character, as supported by XPS analysis. Overall, Sm and Sb doping provide an effective route to defect engineering in Co₃O₄, enabling controlled modulation of magnetic interactions for functional device applications.

We have studied non-equilibrium states in NbTi films deposited on fused silica under different surrounding conditions using electrical current pulses. When the applied current exceeds the depairing current (I_{c}), a voltage response is observed after a characteristic delay time (t_{d}). Although the sample dimensions are much larger than the coherence length (xi), the localized micro-metric region formed corresponds to a hotspot. The delay times are quantitatively described using the Time-Dependent Ginzburg–Landau (TDGL) theory of M. Tinkham, enabling the determination of filament cooling times for the same sample in both liquid helium and vacuum. We find cooling times of about (tau _{d} = 6.0 pm 0.5) ns in vacuum and (5.3 pm 0.5) ns in liquid helium. These results demonstrate that liquid helium has only a minor effect on the film cooling time, with most of the heat being transferred into the substrate via phonons.

The paper considers micromagnetic structures arising in a uniaxial disk-shaped ferromagnet containing columnar “potential well” type defect located at its center, within which the material parameters are altered. It was shown that in the disk (in the absence of a defect), (:kpi:)-skyrmions ((:k=1,:2,dots:7)) are alternately stabilized depending on the defect radius. It was found that in this case, the transformation of the disk’s micromagnetic structure from a state with (:kpi:)-skyrmions occurs through the widening of its rings and the formation of a transition region at the disk edge, in which the magnetization vector once again rotates by 180°. Taking into account the presence of the defect, skyrmion stabilization occurs according to a different scenario: the transformation of (:kpi:)-skyrmions into (:(k+1)pi:)-skyrmions in the central region of the disk, corresponding to the core. In addition, it was found that the critical disk dimensions at which these transformations occur are significantly lower than in the absence of a defect in the disk, which potentially simplifies the production of more complex skyrmions in structure.

The band structure, orbital contributions to the spectral weight of the states at the top of the valence band, and the type of band gap in a CuO monolayer are investigated for different values of the hopping integrals and the Coulomb interactions on copper, (U'_d), and oxygen, (U'_p). The electronic structure of quasiparticle excitations is calculated using an eight-band p–d model that includes all Cu 3d and O 2p orbitals, together with the generalized tight-binding method and the the equations of motion method for Green’s functions of Hubbard operators within the multiband Hubbard model. The energy spectrum of local states and dispersionless quasiparticle excitations is obtained. The ratio ({{U'_d}}/ {{U'_p}}) determines both the magnitude and the type of the band gap. Depending on this ratio, Mott–Hubbard (MH), charge-transfer (CT), or interorbital band gap can arise. When (U'_d) dominates over (U'_p), the system is insulating, with a band gap opening between d-orbitals. A metallic state can be realized only when (U'_p) exceeds (U'_d). The metal–insulator transition driven by increasing hopping integrals is possible only for (U'_d < 1.5U'_p).

This study investigates the superconducting properties of Dy-intercalated bilayer graphene ((DyC_6)) using first-principles calculations. Under an applied pressure of 2 GPa, the electronic structure and electron-phonon coupling of (DyC_6) are evaluated within the EPW module of Quantum ESPRESSO. The results reveal a metallic multi-band Fermi surface with several bands crossing the Fermi level, indicative of strong electron-phonon coupling. The calculated electron-phonon coupling constant is (lambda )= 0.929, and the superconducting critical temperature is predicted to be (Tc approx ) 11.14 K, with a superconducting gap of 1.69 meV. Analysis of the Eliashberg spectral function shows that superconductivity arises from the synergistic contribution of low-frequency Dy-related phonon modes and high-frequency C-related modes. Despite the inherent magnetic moment of Dy, calculations indicate that its magnetism is nearly quenched in the (DyC_6) system due to hybridization with carbon layers, allowing BCS-type phonon-mediated superconductivity to emerge. This work predicts (DyC_6) as a promising graphene-based superconductor and provides a theoretical basis for integrating magnetic rare-earth elements into two-dimensional materials to engineer superconducting properties.

This study employs Monte Carlo simulations to investigate the influence of geometry, linear and biquadratic coupling interactions and temperature on the magnetic hysteresis properties of Husimi nanostructures. By analyzing triangular, square and pentagonal Husimi geometries, we demonstrate how these factors govern coercive fields, hysteresis loop behavior and thermal stability. The findings provide valuable insights into designing nanostructures tailored for technological applications, such as magnetic storage, memory devices, and spintronic sensors, emphasizing the role of geometry and coupling interactions in optimizing magnetic performance.

In this study, high-quality glass-coated amorphous microwires (Co_0.7Fe_0.05Si_0.15B_0.1)_100−xCu_x (x = 0, 1, and 2) with few impurity phases, good continuity, and uniformity were successfully prepared using an optimized high-frequency induction melt-drawing technology. This work systematically investigated the effects of the direct current (DC) annealing process on the magnetic properties and giant magnetoimpedance (GMI) effect of these microwires. It was found that Cu doping enhances the crystallization stability of the alloy to a certain extent, and an appropriate content of the Cu element can significantly enhance the GMI effect of the microwires. The nanocrystallization mechanism discussed later is inferred based on relevant literature reports.What’s more, the DC annealing not only remarkably reduces the coercivity (H_c) of the microwires, but also significantly improves their saturation magnetization (M_s) and GMI effect. After DC annealing with 25 mA current, the H_c of (Co_0.7Fe_0.05Si_0.15B_0.1)₉₈Cu₂ microwires decreased from 3.2 A/m to 1.69 A/m, and the M_s increased from 50.49 emu/g to 73.03 emu/g. Meanwhile, by optimizing Cu content and DC annealing parameters, this work demonstrates a highly effective route to achieving a GMI ratio of 687% in Co-based microwires, offering a practical processing strategy for advanced GMI sensor materials.

We apply to underdoped cuprates our FL* (fractionalized Fermi Liquid) formalism, where in addition to the standard holes other quasi-particle excitations are (semionic) holons that only carry charge and gapped spinons that only carry spin, interacting via a slave-particle gauge field. We first apply this approach to the “pseudogap phase” and we show that the slave-particle gauge interaction binds the holon and the spinon into a hole resonance whose spectral weight is essentially concentrated on Fermi arcs. Furthermore the self-energy of the hole exhibits what we call a Marginal FL* behaviour, i.e. the relaxation rate of the hole at zero temperature and small energy is proportional to the absolute value of the energy, measured from the Fermi surface. Along the nodal direction such behaviour combined with an additional Fermi liquid contribution is shown to be in agreement with many experimental data. We then apply the FL* formalism in the superconducting region of the doping-temperature phase diagram below the "pseudogap phase". There we show the appearance of a magnetic excitation, that we call magnon, as a spinon-antispinon resonance whose spectral weight exhibits a hourglass structure, qualitatively in agreement with neutron experiments. This structure appears as a consequence of a finite density of RVB spinon pairs and the slave-particle gauge attraction between unpaired spinons and antispinons.